Refrigeration System with Combined Superheat and Subcooling Control

ABSTRACT

A controller for a refrigeration system includes a processing circuit having one or more processors and memory. The processing circuit is configured to calculate a superheat of a gas refrigerant exiting a first side of a subcooler based on a measured temperature and a measured pressure of the gas refrigerant and compare the calculated superheat to a superheat threshold. In response to a determination that the calculated superheat is less than the superheat threshold, the processing circuit closes an expansion valve to restrict a flow of the gas refrigerant through a second side of the subcooler. In response to a determination that the calculated superheat is equal to or greater than the superheat threshold, the processing circuit operates the expansion valve to drive a temperature of a subcooled liquid refrigerant exiting the second side of the subcooler to a subcooled liquid temperature setpoint.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 17/552,180, filed on Dec. 15, 2021, which is a divisionalapplication of U.S. patent application Ser. No. 16/126,275, filed onSep. 10, 2018, now U.S. Pat. No. 11,243,016, which in turn claimspriority to U.S. Provisional Patent Application Ser. No. 62/557,478,filed on Sep. 12, 2017, the entire contents of each of which areincorporated herein by reference.

BACKGROUND

The present disclosure relates generally to a refrigeration system andmore particularly to a refrigeration system with a subcooler configuredto subcool a liquid refrigerant.

Some refrigeration systems include a subcooler configured to subcool aliquid refrigerant. The subcooled liquid refrigerant can be expanded andprovided to one or more evaporators of the refrigeration system. Thesubcooler may transfer heat from the liquid refrigerant to a gasrefrigerant. Conventional subcoolers include an expansion valveconfigured to control the flow of the gas refrigerant through thesubcooler and a separate pressure regulator valve configured to controlthe superheat of the gas refrigerant exiting the subcooler. It would bedesirable to simplify the operation of the refrigeration system whilemaintaining the ability to provide both superheat control and subcoolingcontrol.

SUMMARY

A refrigeration system includes a subcooler configured to providesubcooling for a liquid refrigerant flowing through a first side of thesubcooler by transferring heat from the liquid refrigerant to a gasrefrigerant flowing through a second side of the subcooler. An expansionvalve is located at an inlet of the second side of the subcooler andconfigured to control a flow of the gas refrigerant through the secondside of the subcooler. A gas temperature sensor and a gas pressuresensor are configured to measure a temperature and pressure of the gasrefrigerant. A liquid temperature sensor is configured to measure atemperature of the subcooled liquid refrigerant. A controller isconfigured to calculate a superheat of the gas refrigerant based on themeasured temperature and measured pressure of the gas refrigerant andmay compare the calculated superheat to a superheat threshold. If thecalculated superheat is less than the superheat threshold, thecontroller may close the expansion valve. However, if the calculatedsuperheat is equal to or greater than the superheat threshold, thecontroller may operate the expansion valve using a feedback controltechnique to drive the temperature of the subcooled liquid refrigerantto a subcooled liquid temperature setpoint.

In some embodiments, the refrigeration system includes a fluid conduitcoupled to an outlet of the second side of the subcooler and configuredto receive the gas refrigerant exiting the subcooler. The gastemperature sensor and the gas pressure sensor may be located along thefluid conduit and configured to measure the temperature and pressure ofthe gas refrigerant within the fluid conduit.

In some embodiments, the refrigeration system includes a liquidrefrigerant line coupled to an inlet of the first side of the subcoolerand configured to deliver a first portion of the liquid refrigerant tothe first side of the subcooler. In some embodiments, the refrigerationsystem includes a connecting line coupled to the liquid refrigerant lineand to the expansion valve and configured to deliver a second portion ofthe liquid refrigerant to the expansion valve.

In some embodiments, the controller includes a valve controllerconfigured to operate the expansion valve, a feedback controllerconfigured to generate a valve position signal using the feedbackcontrol technique and provide the valve position signal to the valvecontroller, and a comparator configured to compare the calculatedsuperheat to a superheat threshold and provide a valve close signal thevalve controller in response to a determination that the calculatedsuperheat is less than the superheat threshold.

In some embodiments, the valve controller is configured to operate theexpansion valve using the valve position signal when the close signal isnot provided by the comparator and override the valve position signalwith the close signal when the close signal is provided by thecomparator, the close signal causing the expansion valve to close.

In some embodiments, the feedback controller is configured to generatethe valve position signal based on a difference between the temperatureof the subcooled liquid refrigerant and the subcooled liquid temperaturesetpoint.

In some embodiments, closing the expansion valve includes completelyclosing the expansion valve to completely stop a flow of the gasrefrigerant through the subcooler or partially closing the expansionvalve to reduce the flow of the gas refrigerant through the subcooler.

Another implementation of the present disclosure is controller for arefrigeration system. The controller includes a processing circuithaving one or more processors and memory. The processing circuit isconfigured to calculate a superheat of a gas refrigerant exiting asecond side of a subcooler based on a measured temperature and ameasured pressure of the gas refrigerant and compare the calculatedsuperheat to a superheat threshold. In response to a determination thatthe calculated superheat is less than the superheat threshold, theprocessing circuit may close an expansion valve located at an inlet ofthe second side of the subcooler and to restrict a flow of the gasrefrigerant through the second side of the subcooler. In response to adetermination that the calculated superheat is equal to or greater thanthe superheat threshold, the processing circuit may operate theexpansion valve using a feedback control technique to drive atemperature of a subcooled liquid refrigerant exiting a first side ofthe subcooler to a subcooled liquid temperature setpoint.

In some embodiments, the temperature and pressure of the gas refrigerantare measured by a gas temperature sensor and a gas pressure sensorlocated along a fluid conduit coupled to an outlet of the second side ofthe subcooler and configured to receive the gas refrigerant exiting thesubcooler.

In some embodiments, the processing circuit includes a valve controllerconfigured to operate the expansion valve, a feedback controllerconfigured to generate a valve position signal using the feedbackcontrol technique and provide the valve position signal to the valvecontroller, and a comparator configured to compare the calculatedsuperheat to a superheat threshold and provide a valve close signal thevalve controller in response to a determination that the calculatedsuperheat is less than the superheat threshold.

In some embodiments, the valve controller is configured to operate theexpansion valve using the valve position signal when the close signal isnot provided by the comparator and override the valve position signalwith the close signal when the close signal is provided by thecomparator, the close signal causing the expansion valve to close.

In some embodiments, the feedback controller is configured to generatethe valve position signal based on a difference between the temperatureof the subcooled liquid refrigerant and the subcooled liquid temperaturesetpoint.

In some embodiments, closing the expansion valve includes completelyclosing the expansion valve to completely stop a flow of the gasrefrigerant through the subcooler or partially closing the expansionvalve to reduce the flow of the gas refrigerant through the subcooler.

Another implementation of the present disclosure is a method forcontrolling a refrigeration system. The method includes operating asubcooler to provide subcooling for a liquid refrigerant flowing througha first side of the subcooler by transferring heat from the liquidrefrigerant to a gas refrigerant flowing through a second side of thesubcooler, operating an expansion valve located at an inlet of thesecond side of the subcooler to control a flow of the gas refrigerantthrough the second side of the subcooler, measuring a temperature andpressure of the gas refrigerant using a gas temperature sensor and a gaspressure sensor, measuring a temperature of the subcooled liquidrefrigerant using a liquid temperature sensor, calculating a superheatof the gas refrigerant based on the measured temperature and measuredpressure of the gas refrigerant, and comparing the calculated superheatto a superheat threshold. In response to a determination that thecalculated superheat is less than the superheat threshold, the methodincludes closing the expansion valve. In response to a determinationthat the calculated superheat is equal to or greater than the superheatthreshold, the method includes operating the expansion valve using afeedback control technique to drive the temperature of the subcooledliquid refrigerant to a subcooled liquid temperature setpoint.

In some embodiments, the method includes receiving the gas refrigerantexiting the subcooler within a fluid conduit coupled to an outlet of thesecond side of the subcooler. The gas temperature sensor and the gaspressure sensor may be located along the fluid conduit and measure thetemperature and pressure of the gas refrigerant within the fluidconduit.

In some embodiments, the method includes delivering a first portion ofthe liquid refrigerant to the first side of the subcooler via a liquidrefrigerant line coupled to an inlet of the first side of the subcoolerand delivering a second portion of the liquid refrigerant to theexpansion valve via a connecting line coupled to the liquid refrigerantline and to the expansion valve.

In some embodiments, the expansion valve is operated by a valvecontroller. The method may further include generating a valve positionsignal using the feedback control technique and providing the valveposition signal to the valve controller and providing a valve closesignal the valve controller in response to a determination that thecalculated superheat is less than the superheat threshold.

In some embodiments, operating the expansion valve includes operatingthe expansion valve using the valve position signal when the closesignal is not provided and overriding the valve position signal with theclose signal when the close signal is provided, the close signal causingthe expansion valve to close.

In some embodiments, the valve position signal is generated based on adifference between the temperature of the subcooled liquid refrigerantand the subcooled liquid temperature setpoint.

In some embodiments, closing the expansion valve includes completelyclosing the expansion valve to completely stop a flow of the gasrefrigerant through the subcooler or partially closing the expansionvalve to reduce the flow of the gas refrigerant through the subcooler.

Those skilled in the art will appreciate that the summary isillustrative only and is not intended to be in any way limiting. Otheraspects, inventive features, and advantages of the devices and/orprocesses described herein, as defined solely by the claims, will becomeapparent in the detailed description set forth herein and taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is diagram of a refrigeration system with independent superheatand subcooling control, according to an exemplary embodiment.

FIG. 2 is diagram of a refrigeration system with combined superheat andsubcooling control, according to an exemplary embodiment.

FIG. 3 is block diagram illustrating the controller of FIG. 2 in greaterdetail, according to an exemplary embodiment.

FIG. 4 is a flowchart of a process for controlling an expansion valve toprovide combined superheat and subcooling control, according to anexemplary embodiment.

DETAILED DESCRIPTION

Referring generally to the FIGURES, a refrigeration system with combinedsuperheat and subcooling control and components thereof are shown,according to various exemplary embodiments. The refrigeration system mayinclude a subcooler configured to provide subcooling for a liquidrefrigerant delivered to one or more evaporators. The subcooler mayinclude a first side through which a liquid refrigerant flows and asecond side through which a refrigerant gas flows. The refrigerant gasmay absorb heat from the liquid refrigerant in the subcooler to providesubcooling for the liquid refrigerant.

Advantageously, a single electronic expansion valve (EEV) can beoperated to control both the superheat of the gas refrigerant exitingthe second side of the subcooler and the subcooled liquid refrigerantexiting the first side of the subcooler. In some embodiments, atemperature sensor and a pressure sensor are positioned to measure thetemperature and pressure of the refrigerant gas at the outlet of thesecond side of the subcooler and/or within the second side of thesubcooler. A controller can use the temperature and pressuremeasurements to calculate the superheat of the refrigerant gas. Thecontroller may compare the calculated superheat to a superheatthreshold.

If the calculated superheat is less than the superheat threshold, thecontroller may close the EEV (completely or partially) to stop orrestrict the flow of the gas refrigerant through the subcooler. However,if the calculated superheat is equal to or greater than the superheatthreshold, the controller may use a feedback control technique tocontrol the position of the EEV. For example, the controller may receivea measurement of the temperature of the subcooled liquid refrigerant atthe outlet of the first side of the subcooler and may calculate an errorbetween the measured temperature of the subcooled liquid and a subcooledliquid temperature setpoint. The controller may adjust the position ofthe EEV to drive the error to zero. These and other features of therefrigeration system are described in greater detail below.

Refrigeration System with Independent Superheat and Subcooling Control

Referring now to FIG. 1 , a refrigeration system 100 is shown, accordingto an exemplary embodiment. Refrigeration system 100 may be used inconjunction with a temperature-controlled display device (e.g., arefrigerated merchandiser) or other refrigeration device used to storeand/or display refrigerated or frozen objects in a commercial,institutional, or residential setting. Refrigeration system 100 is shownto include condensers 102, a receiver 104, a subcooler 106, expansiondevices 108, evaporators 110, compressors 112, a controller 114, and avariety of sensors and control valves. In some embodiments,refrigeration system 100 operates using a vapor-compressionrefrigeration cycle in which a refrigerant is circulated betweencondensers 102 and evaporators 110 to provide cooling for evaporators110.

Condensers 102 may be heat exchangers or other similar devices forremoving heat from a refrigerant that circulates between evaporators 110and condensers 102. In some embodiments, condensers 102 include multiplecondensers arranged in parallel with each other. Condensers 102 mayreceive vapor refrigerant from compressors 112 and may partially orfully condense the vapor refrigerant by removing heat from therefrigerant. The condensation process may result in a liquid refrigerantor a liquid-vapor mixture. In other embodiments, condensers 102 cool therefrigerant vapor (e.g., by removing superheat) without condensing therefrigerant vapor. In some embodiments, the cooling/condensation processis an isobaric process. Condensers 102 may provide the cooled and/orcondensed refrigerant to receiver 104 via fluid conduit 103.

Receiver 104 may collect liquid refrigerant from fluid conduit 103 andmay receive gas refrigerant via fluid conduit 115. In some embodiments,receiver 104 is a flash tank or other fluid reservoir. Receiver 104 mayinclude a liquid portion and a vapor portion and may contain a partiallysaturated mixture of liquid refrigerant and vapor refrigerant. In someembodiments, receiver 104 separates the liquid refrigerant from thevapor refrigerant. The liquid refrigerant may exit receiver 104 throughfluid conduit 105, which may deliver the liquid refrigerant to a firstside 116 of subcooler 106.

Subcooler 106 can be configured to subcool the liquid refrigerantprovided by receiver 104. Subcooler 106 is shown to include a first side116 and a second side 118. First side 116 may receive the liquidrefrigerant from fluid conduit 105. In some embodiments, a connectingline 107 fluidly connects fluid conduit 105 to a second side 118 ofsubcooler 106. An electronic expansion valve (EEV) may be disposed alongconnecting line 107 and may be configured to expand the liquidrefrigerant to a low temperature gas. The gas refrigerant may flowthrough second side 118 of subcooler 106 and may absorb heat from theliquid refrigerant in first side 116, thereby providing subcooling forthe liquid refrigerant. The subcooled refrigerant exiting first side 116may be delivered to expansion devices 108 via a fluid conduit 109,whereas the gas refrigerant exiting second side 118 may be delivered tofluid conduit 111 (downstream of evaporators 110) via a fluid conduit117.

Expansion devices 108 may be electronic expansion valves or anothersimilar expansion devices. In various embodiments, expansion devices 108may be controlled by controller 114 (e.g., using an automatic controlscheme), manually by a user, or may be set to a predetermined position.In some embodiments, expansion devices 108 include multiple expansiondevices arranged in parallel with each other. Expansion devices 108 maycause the subcooled refrigerant to undergo a rapid drop in pressure,thereby expanding the subcooled refrigerant to a lower pressure, lowertemperature state. The expanded refrigerant is then provided toevaporators 110.

Evaporators 110 may receive the cooled and expanded refrigerant fromexpansion devices 108. In some embodiments, evaporators 110 includemultiple evaporators arranged in parallel with each other. In someembodiments, evaporators 110 are associated with display cases/devices(e.g., if refrigeration system 100 is implemented in a supermarketsetting). Evaporators 110 may be configured to facilitate the transferof heat from the display cases/devices into the refrigerant. The addedheat may cause the refrigerant to evaporate partially or completely. Insome embodiments, the evaporation process may be an isobaric process.Evaporators 110 may output the refrigerant into a suction line 111 whichdelivers the refrigerant to compressors 112.

Compressors 112 may compress the refrigerant to a high temperature, highpressure gas. In some embodiments, compressors 112 include multiplecompressors arranged in parallel with each other. Compressors 112 candischarge the high temperature, high pressure refrigerant into adischarge line 113 which fluidly connects compressors 112 to condensers102. In some embodiments, a fluid conduit 115 fluidly connects dischargeline 113 to a gas inlet of receiver 104. Compressors 112 may becontrolled by controller 114, or by any suitable controller and controlscheme.

Refrigeration system 100 is shown to include a variety of sensors andcontrol valves. For example, refrigeration system 100 may include aliquid pressure sensor PL1 and a liquid temperature sensor TL1configured to measure the pressure and temperature of the liquidrefrigerant in fluid conduit 103. In some embodiments, the measurementsobtained by sensors PL1 and TL1 are provided as an input to controller114. Controller 114 can use the measurements obtained by sensors PL1 andTL1 to control the position of the electronic pressure regulator valveEFPR positioned along fluid conduit 103. The EFPR valve can be operatedby controller 114 to regulate the pressure of the refrigerant inreceiver 104.

Refrigeration system 100 may include a receiver pressure sensor PR1configured to measure the pressure of the refrigerant in fluid conduit115 and/or within receiver 104. In some embodiments, the measurementsobtained by pressure sensor PR1 are provided as an input to controller114. Controller 114 can use the measurements obtained by pressure sensorPR1 to control the position of the electronic pressure regulator valveERPR positioned along fluid conduit 115. The ERPR valve can be operatedby controller 114 to regulate the pressure of the refrigerant gas inreceiver 104.

Refrigeration system 100 may include an ambient temperature sensor TM1configured to measure the ambient temperature outside condensers 102. Insome embodiments, the measurements obtained by temperature sensor TM1are provided as an input to controller 114. Controller 114 can use themeasurements obtained by temperature sensor TM1 determine a temperaturedifferential between the refrigerant in condensers 102 and the ambienttemperature. This temperature differential may have an impact on therate of heat transfer provided by condensers 102 and can be used bycontroller 114 to determine the operating efficiency of condensers 102.

Refrigeration system 100 may include a gas pressure sensor PS11 and agas temperature sensor TS11 configured to measure the pressure andtemperature of the gas refrigerant exiting second side 118 of subcooler106 (i.e., the gas refrigerant in fluid conduit 117). Alternatively,sensors P511 and TS11 can be positioned to measure the pressure andtemperature of the refrigerant gas within second side 118 of subcooler106. In some embodiments, the measurements obtained by sensors PS11 andTS11 are provided as an input to controller 114. Controller 114 can usethe measurements obtained by sensors PS11 and TS11 to calculate anamount of superheat of the gas refrigerant in fluid conduit 117. In someembodiments, controller 114 uses the calculated amount of superheat tocontrol the position of an electronic expansion valve (EEV) positionedalong connecting line 107. Controller 114 can variably open and closethe EEV to regulate the amount of superheat of the gas refrigerantwithin second side 118 of subcooler 106 and/or within fluid conduit 117.Controller 114 can set the position of the EEV to a fully open position,a fully closed position, or any intermediate position between the fullyopen and fully closed positions to adjust the amount of superheat. Inother embodiments, the measurements obtained by sensors P511 and TS11are provided directly to the EEV and used by the EEV to calculate theamount of superheat of the gas refrigerant in fluid conduit 117. The EEVcan use a feedback control technique (e.g., PID control) to open andclose the EEV to control the amount of superheat.

In some embodiments, adjusting the EEV toward the fully closed positiondecreases the pressure of the refrigerant gas in subcooler 106, whichincreases the amount of superheat. Conversely, adjusting the EEV towardthe fully open position may increase the pressure of the refrigerant gaswithin subcooler 106, which decreases the amount of superheat.Accordingly, controller 114 can variably close the EEV (i.e., move theEEV toward the fully closed position) to increase the amount ofsuperheat in response to a determination that the calculated amount ofsuperheat is less than a setpoint or target value. Similarly, controller114 can variably open the EEV (i.e., move the EEV toward the fully openposition) to decrease the amount of superheat in response to adetermination that the calculated amount of superheat is greater thanthe setpoint or target value. In this way, controller 114 can controlthe EEV to achieve a setpoint or target value for the amount ofsuperheat of the gas refrigerant exiting subcooler 106.

Refrigeration system 100 may include a gas pressure sensor PS11Bconfigured to measure the pressure of the gas refrigerant exiting secondside 118 of subcooler 106 (i.e., the gas refrigerant in fluid conduit117). Alternatively, sensor PS11B can be positioned to measure thepressure of the refrigerant gas within second side 118 of subcooler 106.In some embodiments, the measurements obtained by sensor PS11B areprovided as an input to controller 114. Controller 114 can use themeasurements obtained by sensor P511B to control the position of anelectronic pressure regulator valve (EEPR) positioned along fluidconduit 117. Controller 114 can variably open and close the EEPR valveto regulate the pressure of the gas refrigerant within second side 118of subcooler 106 and/or within fluid conduit 117. Controller 114 can setthe position of the EEPR valve to a fully open position, a fully closedposition, or any intermediate position between the fully open and fullyclosed positions to adjust the pressure of the gas refrigerant. In otherembodiments, the measurements obtained by sensor P511B are provideddirectly to the EEPR valve and used by the EEPR valve to control thepressure of the gas refrigerant in fluid conduit 117. The EEPR valve canuse a feedback control technique (e.g., PID control) to open and closethe EEPR valve to control the pressure of the gas refrigerant.

Refrigeration system 100 may include a subcooled liquid temperaturesensor TS12 configured to measure the temperature of the subcooledliquid refrigerant exiting first side 116 of subcooler 106. In someembodiments, the measurements obtained by subcooled liquid temperaturesensor TL12 are provided as an input to controller 114. Controller 114can use the measurements obtained by sensor TL12 to control the positionof an electronic expansion valve (EEV) positioned along connecting line107 (e.g., at the inlet of second side 118 of receiver 106). Controller114 can variably open and close the EEV to regulate the amount ofsubcooling applied to the liquid refrigerant within subcooler 106.Controller 114 can set the position of the EEV to a fully open position,a fully closed position, or any intermediate position between the fullyopen and fully closed positions to adjust the amount of subcooling.

In some embodiments, adjusting the EEV toward the fully closed positiondecreases the amount of subcooling (i.e., increases the temperature ofthe subcooled refrigerant), whereas adjusting the EEV toward the fullyopen position increases the amount of subcooling (i.e., decreases thetemperature of the subcooled refrigerant). Accordingly, controller 114can variably close the EEV (i.e., move the EEV toward the fully closedposition) to decrease the subcooling provided by subcooler 106 inresponse to a determination that the measured temperature of thesubcooled refrigerant is less than a setpoint or target value.Similarly, controller 114 can variably open the EEV (i.e., move the EEVtoward a fully open position) to increase the amount of subcoolingprovided by subcooler 106 in response to a determination that themeasured temperature of the subcooled refrigerant is greater than asetpoint or target value. In this way, controller 114 can control theEEV to achieve a setpoint or target value for the temperature of thesubcooled liquid refrigerant exiting subcooler 106.

Refrigeration system 100 may include a liquid pressure sensor PL2 and aliquid temperature sensor TL10 configured to measure the pressure andtemperature of the liquid refrigerant in fluid conduit 109. In someembodiments, the measurements obtained by sensors PL2 and TL10 areprovided as an input to controller 114. Controller 114 can use themeasurements obtained by sensors PL2 and TL10 to control the position ofthe electronic pressure regulator valve ELPR positioned along fluidconduit 109. The ELPR valve can be operated by controller 114 toregulate the pressure of the liquid refrigerant in subcooler 106 and/orin fluid conduit 109.

Refrigeration system 100 may include a suction line pressure sensor PS1and a suction line temperature sensor TS1 configured to measure thepressure and temperature of the gas refrigerant in suction line 111. Insome embodiments, the measurements obtained by sensors PS1 and TS1 areprovided as an input to controller 114. Controller 114 can use themeasurements obtained by sensors PS1 and TS1 to control compressors 112and/or the positions of expansion devices 108. Compressors 112 can beoperated by controller 114 to regulate the pressure of the gasrefrigerant in suction line 111.

Refrigeration system 100 may include a discharge line pressure sensorPD1 and a discharge line temperature sensor TD1 configured to measurethe pressure and temperature of the compressed gas refrigerant indischarge line 113. In some embodiments, the measurements obtained bysensors PD1 and TD1 are provided as an input to controller 114.Controller 114 can use the measurements obtained by sensors PD1 and TD1to control compressors 112. Compressors 112 can be operated bycontroller 114 to regulate the pressure and/or temperature of thecompressed gas refrigerant in discharge line 113.

Refrigeration System with Combined Superheat and Subcooling Control

Referring now to FIG. 2 , another refrigeration system 200 is shown,according to an exemplary embodiment. Refrigeration system 200 may beused in conjunction with a temperature-controlled display device (e.g.,a refrigerated merchandiser) or other refrigeration device used to storeand/or display refrigerated or frozen objects in a commercial,institutional, or residential setting. Refrigeration system 200 mayinclude many of the same components as refrigeration system 100. Forexample, refrigeration system 200 is shown to include condensers 202, areceiver 204, a subcooler 206, expansion devices 208, evaporators 210,compressors 212, a controller 214, and a variety of sensors and controlvalves. In some embodiments, refrigeration system 200 operates using avapor-compression refrigeration cycle in which a refrigerant iscirculated between condensers 202 and evaporators 210 to provide coolingfor evaporators 210.

Condensers 202 may be heat exchangers or other similar devices forremoving heat from a refrigerant that circulates between evaporators 210and condensers 202. In some embodiments, condensers 202 include multiplecondensers arranged in parallel with each other. Condensers 202 mayreceive vapor refrigerant from compressors 212 and may partially orfully condense the vapor refrigerant by removing heat from therefrigerant. The condensation process may result in a liquid refrigerantor a liquid-vapor mixture. In other embodiments, condensers 202 cool therefrigerant vapor (e.g., by removing superheat) without condensing therefrigerant vapor. In some embodiments, the cooling/condensation processis an isobaric process. Condensers 202 may provide the cooled and/orcondensed refrigerant to receiver 204 via fluid conduit 203.

Receiver 204 may collect liquid refrigerant from fluid conduit 203 andmay receive gas refrigerant via fluid conduit 215. In some embodiments,receiver 204 is a flash tank or other fluid reservoir. Receiver 204 mayinclude a liquid portion and a vapor portion and may contain a partiallysaturated mixture of liquid refrigerant and vapor refrigerant. In someembodiments, receiver 204 separates the liquid refrigerant from thevapor refrigerant. The liquid refrigerant may exit receiver 204 throughfluid conduit 205, which may deliver the liquid refrigerant to a firstside 216 of subcooler 206.

Subcooler 206 can be configured to subcool the liquid refrigerantprovided by receiver 204. Subcooler 206 is shown to include a first side216 and a second side 218. First side 216 may receive the liquidrefrigerant from fluid conduit 205. In some embodiments, a connectingline 207 fluidly connects fluid conduit 205 to a second side 218 ofsubcooler 206. An electronic expansion valve (EEV) may be disposed alongconnecting line 207 and may be configured to expand the liquidrefrigerant to a low temperature gas. The gas refrigerant may flowthrough second side 218 of subcooler 206 and may absorb heat from theliquid refrigerant in first side 216, thereby providing subcooling forthe liquid refrigerant. The subcooled refrigerant exiting first side 216may be delivered to expansion devices 208 via a fluid conduit 209,whereas the gas refrigerant exiting second side 218 may be delivered tofluid conduit 211 (downstream of evaporators 210) via a fluid conduit217.

Expansion devices 208 may be electronic expansion valves or anothersimilar expansion devices. In various embodiments, expansion devices 208may be controlled by controller 214 (e.g., using an automatic controlscheme), manually by a user, or may be set to a predetermined position.In some embodiments, expansion devices 208 include multiple expansiondevices arranged in parallel with each other. Expansion devices 208 maycause the subcooled refrigerant to undergo a rapid drop in pressure,thereby expanding the subcooled refrigerant to a lower pressure, lowertemperature state. The expanded refrigerant is then provided toevaporators 210.

Evaporators 210 may receive the cooled and expanded refrigerant fromexpansion devices 208. In some embodiments, evaporators 210 includemultiple evaporators arranged in parallel with each other. In someembodiments, evaporators 210 are associated with display cases/devices(e.g., if refrigeration system 200 is implemented in a supermarketsetting). Evaporators 210 may be configured to facilitate the transferof heat from the display cases/devices into the refrigerant. The addedheat may cause the refrigerant to evaporate partially or completely. Insome embodiments, the evaporation process may be an isobaric process.Evaporators 210 may output the refrigerant into a suction line 211 whichdelivers the refrigerant to compressors 212.

Compressors 212 may compress the refrigerant to a high temperature, highpressure gas. In some embodiments, compressors 212 include multiplecompressors arranged in parallel with each other. Compressors 212 candischarge the high temperature, high pressure refrigerant into adischarge line 213 which fluidly connects compressors 212 to condensers202. In some embodiments, a fluid conduit 215 fluidly connects dischargeline 213 to a gas inlet of receiver 204. Compressors 212 may becontrolled by controller 214, or by any suitable controller and controlscheme.

Refrigeration system 200 is shown to include a variety of sensors andcontrol valves. For example, refrigeration system 200 may include aliquid pressure sensor PL1 and a liquid temperature sensor TL1configured to measure the pressure and temperature of the liquidrefrigerant in fluid conduit 203. In some embodiments, the measurementsobtained by sensors PL1 and TL1 are provided as an input to controller214. Controller 214 can use the measurements obtained by sensors PL1 andTL1 to control the position of the electronic pressure regulator valveEFPR positioned along fluid conduit 203. The EFPR valve can be operatedby controller 214 to regulate the pressure of the refrigerant inreceiver 204.

Refrigeration system 200 may include a receiver pressure sensor PR1configured to measure the pressure of the refrigerant in fluid conduit215 and/or within receiver 204. In some embodiments, the measurementsobtained by pressure sensor PR1 are provided as an input to controller214. Controller 214 can use the measurements obtained by pressure sensorPR1 to control the position of the electronic pressure regulator valveERPR positioned along fluid conduit 215. The ERPR valve can be operatedby controller 214 to regulate the pressure of the refrigerant gas inreceiver 204.

Refrigeration system 200 may include an ambient temperature sensor TM1configured to measure the ambient temperature outside condensers 202. Insome embodiments, the measurements obtained by temperature sensor TM1are provided as an input to controller 214. Controller 214 can use themeasurements obtained by temperature sensor TM1 determine a temperaturedifferential between the refrigerant in condensers 202 and the ambienttemperature. This temperature differential may have an impact on therate of heat transfer provided by condensers 202 and can be used bycontroller 214 to determine the operating efficiency of condensers 202.

Refrigeration system 200 may include a gas pressure sensor PS11 and agas temperature sensor TS11 configured to measure the pressure andtemperature of the gas refrigerant exiting second side 218 of subcooler206 (i.e., the gas refrigerant in fluid conduit 217). Alternatively,sensors P511 and TS11 can be positioned to measure the pressure andtemperature of the refrigerant gas within second side 218 of subcooler206. In some embodiments, the measurements obtained by sensors PS11 andTS11 are provided as an input to controller 214. Controller 214 can usethe measurements obtained by sensors P511 and TS11 to calculate anamount of superheat of the gas refrigerant in fluid conduit 217. In someembodiments, controller 214 uses the calculated amount of superheat tocontrol the position of an electronic expansion valve (EEV) positionedalong connecting line 207. For example, controller 214 can be configuredto close the EEV in response to a determination that the calculatedamount of superheat is less than a threshold value.

Refrigeration system 200 may include a subcooled liquid temperaturesensor TS12 configured to measure the temperature of the subcooledliquid refrigerant exiting first side 216 of subcooler 206. In someembodiments, the measurements obtained by subcooled liquid temperaturesensor TL12 are provided as an input to controller 214. Controller 214can use the measurements obtained by sensor TL12 to control the positionof the EEV positioned along connecting line 207 (e.g., at the inlet ofsecond side 218 of receiver 206). Controller 214 can variably open andclose the EEV to regulate the amount of subcooling applied to the liquidrefrigerant within subcooler 206. Controller 214 can set the position ofthe EEV to a fully open position, a fully closed position, or anyintermediate position between the fully open and fully closed positionsto adjust the amount of subcooling.

In some embodiments, adjusting the EEV toward the fully closed positiondecreases the amount of subcooling (i.e., increases the temperature ofthe subcooled refrigerant), whereas adjusting the EEV toward the fullyopen position increases the amount of subcooling (i.e., decreases thetemperature of the subcooled refrigerant). Accordingly, controller 214can variably close the EEV (i.e., move the EEV toward the fully closedposition) to decrease the subcooling provided by subcooler 206 inresponse to a determination that the measured temperature of thesubcooled refrigerant is less than a setpoint or target value.Similarly, controller 214 can variably open the EEV (i.e., move the EEVtoward a fully open position) to increase the amount of subcoolingprovided by subcooler 206 in response to a determination that themeasured temperature of the subcooled refrigerant is greater than asetpoint or target value. In this way, controller 214 can control theEEV to achieve a setpoint or target value for the temperature of thesubcooled liquid refrigerant exiting subcooler 206.

In some embodiments, controller 214 operates the EEV to control both thetemperature of the subcooled liquid in fluid conduit 209 and the amountof superheat of the gas refrigerant in fluid conduit 217. For example,controller 214 can operate the EEV using a feedback control technique(e.g., PID control) to regulate the temperature of the subcooled liquidsubject to a requirement that the amount of superheat of the gasrefrigerant be maintained at or above a predetermined superheatthreshold. If the calculated amount of superheat is less than thesuperheat threshold, controller 214 may close the EEV regardless of thetemperature of the subcooled liquid. However, if the calculated amountof superheat is equal to or greater than the superheat threshold,controller 214 can operate the EEV using a feedback control technique toachieve a target or setpoint value for the temperature of the subcooledliquid. The operations technique performed by controller 214 to controlthe EEV are described in greater detail with reference to FIGS. 3-4 .

Refrigeration system 200 may include a liquid pressure sensor PL2 and aliquid temperature sensor TL10 configured to measure the pressure andtemperature of the liquid refrigerant in fluid conduit 209. In someembodiments, the measurements obtained by sensors PL2 and TL10 areprovided as an input to controller 214. Controller 214 can use themeasurements obtained by sensors PL2 and TL10 to control the position ofthe electronic pressure regulator valve ELPR positioned along fluidconduit 209. The ELPR valve can be operated by controller 214 toregulate the pressure of the liquid refrigerant in subcooler 206 and/orin fluid conduit 209.

Refrigeration system 200 may include a suction line pressure sensor PS1and a suction line temperature sensor TS1 configured to measure thepressure and temperature of the gas refrigerant in suction line 211. Insome embodiments, the measurements obtained by sensors PS1 and TS1 areprovided as an input to controller 214. Controller 214 can use themeasurements obtained by sensors PS1 and TS1 to control compressors 212and/or the positions of expansion devices 208. Compressors 212 can beoperated by controller 214 to regulate the pressure of the gasrefrigerant in suction line 211.

Refrigeration system 200 may include a discharge line pressure sensorPD1 and a discharge line temperature sensor TD1 configured to measurethe pressure and temperature of the compressed gas refrigerant indischarge line 213. In some embodiments, the measurements obtained bysensors PD1 and TD1 are provided as an input to controller 214.Controller 214 can use the measurements obtained by sensors PD1 and TD1to control compressors 212. Compressors 212 can be operated bycontroller 214 to regulate the pressure and/or temperature of thecompressed gas refrigerant in discharge line 213.

Expansion Valve Control

Referring now to FIG. 3 , a block diagram illustrating some componentsof controller 214 in greater detail is shown, according to an exemplaryembodiment. Controller 214 is shown to include a communicationsinterface 302 and a processing circuit 304. Communications interface 302may include wired or wireless interfaces (e.g., jacks, antennas,transmitters, receivers, transceivers, wire terminals, etc.) forconducting data communications with various systems, devices, ornetworks. For example, communications interface 302 may include anEthernet card and port for sending and receiving data via anEthernet-based communications network. In another example,communications interface 302 may include a WiFi transceiver forcommunicating via a wireless communications network. Communicationsinterface 302 may be configured to communicate via local area networksor wide area networks (e.g., the Internet, a building WAN, etc.) and mayuse a variety of communications protocols (e.g., TCP/IP, point-to-point,etc.). In some embodiments, controller 214 uses communications interface302 to receive input from various sensors and send control signals tovarious operable components of refrigeration system 200.

Processing circuit 304 is shown to include a processor 306 and memory308. Processor 306 may be a general purpose or specific purposeprocessor, an application specific integrated circuit (ASIC), one ormore field programmable gate arrays (FPGAs), a group of processingcomponents, or other suitable processing components. Processor 306 maybe configured to execute computer code or instructions stored in memoryor received from other computer readable media (e.g., CDROM, networkstorage, a remote server, etc.).

Memory 308 may include one or more devices (e.g., memory units, memorydevices, storage devices, etc.) for storing data and/or computer codefor completing and/or facilitating the various processes described inthe present disclosure. Memory 308 may include random access memory(RAM), read-only memory (ROM), hard drive storage, temporary storage,non-volatile memory, flash memory, optical memory, or any other suitablememory for storing software objects and/or computer instructions. Memory308 may include database components, object code components, scriptcomponents, or any other type of information structure for supportingthe various activities and information structures described in thepresent disclosure. Memory 308 may be communicably connected toprocessor 306 via processing circuit 304 and may include computer codefor executing one or more processes described herein.

Still referring to FIG. 3 , controller 214 is shown to include asuperheat calculator 310. Superheat calculator 310 can be configured tocalculate an amount of superheat of the gas refrigerant exitingsubcooler 206 (i.e., the gas refrigerant in fluid conduit 217) and/orthe gas refrigerant within second side 218 of subcooler 206. Superheatcalculator 310 may receive measurements of the pressure and temperatureof the gas refrigerant in fluid conduit 217 from pressure sensor PS11and temperature sensor TS11. Sensors PS11 and TS11 can be positioned tomeasure the pressure and temperature of the gas refrigerant withinsecond side 218 of subcooler 206 or at the outlet of subcooler 206(e.g., within fluid conduit 217). Superheat calculator 310 can use themeasured pressure as an input to a function or lookup table to determinethe boiling point of the refrigerant at the measured pressure. Superheatcalculator 310 can subtract the boiling point of the refrigerant fromthe measured temperature to calculate the amount of superheat.

Controller 214 is shown to include a comparator 312 and a valvecontroller 314. Comparator 312 may receive the calculated superheat fromsuperheat calculator 310 and may compare the calculated superheat to asuperheat threshold. If the calculated superheat is less than thesuperheat threshold, comparator 312 may provide a signal to valvecontroller 314 to close the EEV (i.e., a “close signal”). In someembodiments, the close signal operates as an override signal whichcauses valve controller 314 to close the EEV regardless of other inputsto valve controller 314. Closing the EEV may include completely closingthe EEV to completely stop the flow of refrigerant gas through secondside 218 of subcooler 206 or partially closing the EEV to reduce theflow of refrigerant gas through second side 218 of subcooler 206.However, if the calculated superheat is equal to or greater than thesuperheat threshold, comparator 312 may not provide the close signal tovalve controller 314. In the absence of the close signal, valvecontroller 314 may operate the EEV in accordance with a valve positionsignal received from feedback controller 316 and/or a valve commandsignal received from an external data source (e.g., a manual override).

Controller 214 is shown to include a feedback controller 316 and asubtraction element 318. Subtraction element 318 may receivemeasurements of the subcooled liquid refrigerant temperature from thetemperature sensor TL12 located at the subcooled liquid outlet ofsubcooler 206 (i.e., along fluid conduit 209). Subtraction element 318may subtract the measured subcooled liquid temperature from a subcooledliquid temperature setpoint to calculate an error signal. Feedbackcontroller 316 can be configured to control the position of the EEVusing a feedback control technique. For example, feedback controller 316may receive the error signal from subtraction element 318 and maygenerate the valve position signal that drives the error signal to zero.In various embodiments, feedback controller 316 may operate as aproportional-integral (PI) controller, aproportional-integral-derivative (PID) controller, a pattern recognitionadaptive controller (PRAC), a model recognition adaptive controller(MRAC), a model predictive controller (MPC), or any other type offeedback controller using any type of feedback control technique. Insome embodiments, feedback controller 316 provides the valve positionsignal to valve controller 314.

Valve controller 314 may be configured to provide a control signal tothe EEV. The control signal may cause the EEV to open, close, or move toan intermediate position. In some embodiments, valve controller 314 usesa combination of inputs from comparator 312 and feedback controller 316to determine the value of the control signal 316. For example, ifcomparator 312 is providing the close signal, valve controller 314 mayprovide a control signal that causes the EEV to close regardless of thevalue of the valve position signal from feedback controller 316.However, if comparator 312 is not providing the close signal, valvecontroller 314 may use the valve position signal from feedbackcontroller 316 to generate the control signal for the EEV. In someembodiments, valve controller 314 passes the valve position signal fromfeedback controller 316 to the EEV in the absence of the close signalfrom comparator 312, thereby allowing feedback controller 316 to controlthe position of the EEV.

Expansion Valve Control Process

Referring now to FIG. 4 , a flowchart of a process 400 for controllingan electronic expansion valve in a refrigeration system is shown,according to an exemplary embodiment. In some embodiments, process 400is performed by one or more components of refrigeration system 200, asdescribed with reference to FIGS. 2-3 . For example, process 400 can beperformed by controller 214 using input from various sensors.

Process 400 is shown to include measuring temperature and pressure of agas refrigerant that absorbs heat in a subcooler (step 402). Thesubcooler (e.g., subcooler 206) may include a first side (e.g., side216) through which a liquid refrigerant flows and a second side (e.g.,side 218) through which a gas refrigerant flows. The subcooler can beconfigured to transfer heat from the first side to the second side toprovide subcooling for the liquid refrigerant. Accordingly, the gasrefrigerant in the second side may absorb heat from the liquidrefrigerant in the first side (as shown in FIG. 3 ) to providesubcooling for the liquid refrigerant in the first side. In someembodiments, the temperature and pressure are measured by sensors TS11and PS11. The sensors TS11 and PS11 can be positioned at an outlet ofthe subcooler and configured to measure the temperature and pressure ofthe superheated refrigerant gas at the outlet of the subcooler.Alternatively, the sensors TS11 and PS11 can be positioned within thesecond side of the subcooler and configured to measure the temperatureand pressure of the refrigerant gas within the second side of thesubcooler.

Process 400 is shown to include calculating the superheat of the gasrefrigerant based on the temperature and pressure measurements (step404). In some embodiments, step 402 includes using the measured pressureas an input to a function or lookup table to determine the boiling pointof the refrigerant at the measured pressure. The boiling point of therefrigerant at the measured pressure can be subtracted from the measuredtemperature to calculate the amount of superheat. In other words, theamount of superheat can be defined as the amount by which thetemperature of the refrigerant exceeds the boiling point of therefrigerant at the measured pressure.

Process 400 is shown to include determining whether the calculatedsuperheat is less than a superheat threshold (step 406). If thecalculated superheat is less than the superheat threshold (i.e., theresult of step 406 is “yes”), process 400 may proceed to step 408. Step408 may include closing an expansion valve (e.g., the EEV shown in FIGS.2-3 ). In some embodiments, the expansion valve is located at the inletof the second side of the subcooler. The expansion valve can beconfigured to expand the liquid refrigerant to a low temperature, lowpressure gas and provide the refrigerant gas to the second side of thesubcooler. The refrigerant can then flow through the second side of thesubcooler and may absorb heat from the liquid refrigerant flowingthrough the first side of the subcooler to provide subcooling for theliquid refrigerant. In various embodiments, step 408 may involvecompletely closing the expansion valve to completely stop the flow ofrefrigerant gas through the second side of the subcooler or partiallyclosing the expansion valve to reduce the flow of refrigerant gasthrough the second side of the subcooler.

If the calculated superheat is equal to or greater than the superheatthreshold (i.e., the result of step 406 is “no”), process 400 mayproceed to step 410. Step 410 may include measuring the temperature ofthe liquid refrigerant subcooled by the subcooler. In some embodiments,the temperature of the subcooled refrigerant is measured by temperaturesensor TL12. Temperature sensor TL12 can be positioned at the outlet ofthe first side of the subcooler (e.g., along fluid conduit 209) andconfigured to measure the temperature of the subcooled liquidrefrigerant at the outlet of the subcooler. Alternatively, temperaturesensor TL12 can be positioned within the first side of the subcooler andconfigured to measure the temperature of the liquid refrigerant withinthe first side of the subcooler.

Process 400 is shown to include calculating an error between themeasured temperature of the subcooled refrigerant and a subcooledtemperature setpoint (step 412) and operating the expansion valve usinga feedback control technique to drive the error to zero (step 414). Step412 may include subtracting the measured temperature of the subcooledrefrigerant from the subcooled temperature setpoint to calculate theerror. In some embodiments, step 414 includes generating a controlsignal for the expansion valve. The control signal can be generatedusing a proportional-integral (PI) control technique, aproportional-integral-derivative (PID) control technique, a patternrecognition adaptive control (PRAC) technique, a model recognitionadaptive control (MRAC) technique, a model predictive control (MPC)technique, or any other type of feedback control technique. The controlsignal can be provided to the expansion valve, which may cause theexpansion valve to move to a valve position specified by the controlsignal.

Configuration of Exemplary Embodiments

The construction and arrangement of the systems and methods as shown inthe various exemplary embodiments are illustrative only. Although only afew embodiments have been described in detail in this disclosure, manymodifications are possible (e.g., variations in sizes, dimensions,structures, shapes and proportions of the various elements, values ofparameters, mounting arrangements, use of materials, colors,orientations, etc.). For example, the position of elements may bereversed or otherwise varied and the nature or number of discreteelements or positions may be altered or varied. Elements shown asintegrally formed may be constructed of multiple parts or elements. Theelements and assemblies may be constructed from any of a wide variety ofmaterials that provide sufficient strength or durability, in any of awide variety of colors, textures, and combinations. Accordingly, allsuch modifications are intended to be included within the scope of thepresent disclosure. The order or sequence of any process or method stepsmay be varied or re-sequenced according to alternative embodiments.Other substitutions, modifications, changes, and omissions may be madein the design, operating conditions and arrangement of the exemplaryembodiments without departing from the scope of the present disclosure.

Any means-plus-function clause is intended to cover the structuresdescribed herein as performing the recited function and not onlystructural equivalents but also equivalent structures. Additionally, inthe subject description, the word “exemplary” is used to mean serving asan example, instance, or illustration. Any embodiment or designdescribed herein as “exemplary” is not necessarily to be construed aspreferred or advantageous over other embodiments or designs. Rather, useof the word “exemplary” is intended to present concepts in a concretemanner.

The present disclosure contemplates methods, systems and programproducts on any machine-readable media for accomplishing variousoperations. The embodiments of the present disclosure may be implementedusing existing computer processors, or by a special purpose computerprocessor for an appropriate system, incorporated for this or anotherpurpose, or by a hardwired system. Embodiments within the scope of thepresent disclosure include program products comprising machine-readablemedia for carrying or having machine-executable instructions or datastructures stored thereon. Such machine-readable media can be anyavailable media that can be accessed by a general purpose or specialpurpose computer or other machine with a processor. By way of example,such machine-readable media can comprise RAM, ROM, EPROM, EEPROM, CD-ROMor other optical disk storage, magnetic disk storage or other magneticstorage devices, or any other medium which can be used to carry or storedesired program code in the form of machine-executable instructions ordata structures and which can be accessed by a general purpose orspecial purpose computer or other machine with a processor. Combinationsof the above are also included within the scope of machine-readablemedia. Machine-executable instructions include, for example,instructions and data which cause a general purpose computer, specialpurpose computer, or special purpose processing machines to perform acertain function or group of functions.

Although the figures show a specific order of method steps, the order ofthe steps may differ from what is depicted. Also two or more steps maybe performed concurrently or with partial concurrence. Such variationwill depend on the software and hardware systems chosen and on designerchoice. All such variations are within the scope of the disclosure.Likewise, software implementations could be accomplished with standardprogramming techniques with rule based logic and other logic toaccomplish the various connection steps, processing steps, comparisonsteps and decision steps.

1. (canceled)
 2. A method comprising: subcooling a liquid refrigerantflowing through a first side of a subcooler by transferring heat fromthe liquid refrigerant to a gas refrigerant flowing through a secondside of the subcooler; calculating a superheat of the gas refrigerantthat exits the second side of the subcooler based on a measuredtemperature and a measured pressure of the gas refrigerant; comparingthe calculated superheat to a superheat threshold; in response to adetermination that the calculated superheat is less than the superheatthreshold, closing an expansion valve located at an inlet of the secondside of the subcooler to increase the superheat of the gas refrigerantto at or above the superheat threshold; and in response to adetermination that the calculated superheat is equal to or greater thanthe superheat threshold, operating the expansion valve to regulate atemperature of a subcooled liquid refrigerant that exits the first sideof the subcooler to a subcooled liquid temperature setpoint whilemaintaining the superheat of the gas refrigerant at or above thesuperheat threshold.
 3. The method of claim 2, wherein operating theexpansion valve comprises controlling a flow of the gas refrigerantthrough the second side of the subcooler.
 4. The method of claim 2,further comprising measuring a temperature and a pressure of the gasrefrigerant.
 5. The method of claim 2, further comprising measuring atemperature of the subcooled liquid refrigerant.
 6. The method of claim2, further comprising receiving the measured temperature and themeasured pressure of the gas refrigerant from a gas temperature sensorand a gas pressure sensor, respectively, located along a fluid conduitcoupled to an outlet of the second side of the subcooler and configuredto receive the gas refrigerant that exits the subcooler.
 7. The methodof claim 2, further comprising receiving the measured temperature andthe measured pressure of the gas refrigerant from a gas temperaturesensor and a gas pressure sensor, respectively, positioned within thesecond side of the subcooler.
 8. The method of claim 2, furthercomprising: completely closing the expansion valve to completely stop aflow of the gas refrigerant through the subcooler; or partially closingthe expansion valve to reduce the flow of the gas refrigerant throughthe subcooler.
 9. The method of claim 2, further comprising: generatinga valve position signal; providing the valve position signal to theexpansion valve; comparing the calculated superheat to the superheatthreshold; and in response to a determination that the calculatedsuperheat is less than the superheat threshold, providing a valve closesignal to the expansion valve.
 10. The method of claim 9, furthercomprising: operating the expansion valve using the valve positionsignal when the valve close signal is not provided; and overriding thevalve position signal with the valve close signal when the valve closesignal is provided, the valve close signal causing the expansion valveto close.
 11. The method of claim 9, further comprising generating thevalve position signal based on a difference between the temperature ofthe subcooled liquid refrigerant and the subcooled liquid temperaturesetpoint.
 12. The method of claim 9, wherein calculating the superheatof the gas refrigerant based on the measured temperature and themeasured pressure of the gas refrigerant comprises: receiving a value ofthe temperature of the gas refrigerant from a gas temperature sensor;receiving a value of the pressure of the gas refrigerant from a gaspressure sensor; determining a boiling point of the gas refrigerant atthe value of the measured pressure the gas refrigerant; and calculatingan amount of superheat by subtracting the boiling point of the gasrefrigerant from the value of the temperature of the gas refrigerant.13. The method of claim 12, further comprising, wherein determining theboiling point of the gas refrigerant comprises inputting the value ofthe measured pressure of the gas refrigerant into a function or a lookuptable.
 14. The method of claim 9, wherein regulating the temperature ofthe subcooled liquid refrigerant by at least one of aproportional-integral controller, a proportional-integral-derivativecontroller, a pattern recognition adaptive controller, a modelrecognition adaptive controller, or a model predictive controller. 15.The method of claim 9, further comprising receiving the measuredtemperature of the subcooled liquid refrigerant at an outlet of thefirst side of the subcooler from a subcooled liquid temperature sensor.16. The method of claim 15, further comprising: receiving a value of thetemperature of the subcooled liquid refrigerant from the subcooledliquid temperature sensor; calculating an error signal from the value ofthe temperature of the subcooled liquid refrigerant; and transmit theerror signal.
 17. The method of claim 16, wherein calculating the errorsignal from the value of the temperature of the subcooled liquidrefrigerant comprises subtracting the value of the temperature of thesubcooled liquid refrigerant from the subcooled liquid temperaturesetpoint.
 18. The method of claim 17, wherein generating the valveposition signal and providing the valve position signal comprises:receiving the error signal; and generating the valve position signalthat drives the error signal toward zero.
 19. The method of claim 9,further comprising when the valve close signal is not provided,generating a control signal for the expansion valve using the valveposition signal.
 20. The method of claim 9, further comprising when thevalve position signal passes to the expansion valve in an absence of thevalve close signal, controlling a position of the expansion valve. 21.The method of claim 9, further comprising, in an absence of the valveclose signal, operating the expansion valve with the valve positionsignal received from feedback controller or a valve command signalreceived from a manual override.
 22. The method of claim 2, furthercomprising: receiving the measured temperature and the measured pressureof the gas refrigerant from a gas temperature sensor and a gas pressuresensor, respectively, located along a suction line of a plurality ofcompressors fluidly coupled to an outlet of an evaporator, the pluralityof compressors configured to receive a heated refrigerant from theevaporator and increase a pressure of the heated refrigerant to form apressurized heated refrigerant; and operating the plurality ofcompressors to regulate a pressure of the gas refrigerant in the suctionline of the plurality of compressors.
 23. The method of claim 2, furthercomprising: receiving the measured temperature and the measured pressureof the gas refrigerant from a gas temperature sensor and a gas pressuresensor, respectively, located along a discharge line of a plurality ofcompressors fluidly coupled to an outlet of an evaporator, the pluralityof compressors configured to receive a heated refrigerant from theevaporator and increase a pressure of the heated refrigerant to form apressurized heated refrigerant; and operating the plurality ofcompressors to regulate at least one of a pressure or a temperature ofthe gas refrigerant in the discharge line of the plurality ofcompressors.
 24. The method of claim 2, wherein the gas refrigerantcomprises CO₂.